1. Field of the Invention
The present invention relates to a plasma addressing electro-optical device (e.g. liquid crystal display device) of double layer structure composed of an electro-optical cell (e.g. liquid crystal cell ) =and a plasma cell, and more specifically to the structure of the plasma cell for implementing addressing on the basis of a selective plasma discharge.
2. Description of the Related Art
As means for realizing matrix-type liquid crystal display devices of higher resolution and higher contrast, an active matrix addressing technique has been conventionally adopted, by which switching elements such as thin film transistors are provided for display pixels (picture elements), respectively and the respective switching elements are turned on or off in the order of lines. In this prior art technique, however, since a great number of the semiconductor switching elements such as thin film transistors must be formed on a substrates, there exists a problem in that the production yield is deteriorated, in particular when the area or the size of the display device is large.
To overcome the above-mentioned problem, Buzak el al. have proposed a method of adopting plasma switches based upon selective plasma discharge, in place of the switching elements such as thin film transistors, as disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 1-217396, which corresponds to U.S. Pat. Nos. 4,896,149 and 5,077,553, and which are incorporated herein by reference.
The configuration of this prior art plasma-addressing display device for driving the liquid crystal cells by use of the plasma switches will be described briefly hereinbelow with reference to the attached drawings. As shown in FIG. 10, the display device is of layer built structure such that a liquid crystal cell 101 is placed upon a plasma cell 102 via a common dielectric partition 103. The plasma cell 102 is formed by use of a glass substrate 104 including a plurality of parallel arranged channels 105 formed on the surface thereof. The respective channels 105 are closed by the partition 103 so as to form separated plasma chambers 106. The two adjacent plasma chambers 106 are mutually separated by convex portions 107 of the substrate 104, and the respective plasma chambers are filled with an ionizable gas respectively. In addition, a pair of electrodes 108 and 109 are formed on the bottom portions of the respective channels 105 so as to extend in the longitudinal direction thereof. A plurality of pairs of the electrodes function as the anode and cathode electrodes for ionizing the gas within the plasma chambers 106, respectively to generate discharge plasma.
On the other hand, the liquid crystal cell 101 is formed by use of an upper glass substrate 110. The space between the substrate 110 and the partition 103 is filled by a liquid crystal layer 111. On the inner surface of the substrate 110, a plurality of parallel arranged signal electrodes 112 formed by a transparent conductive film are provided in such a way as to intersect the plasma chambers 106. The intersections between the signal electrodes 112 and the plasma chambers 106 correspond to respective pixels arranged in a matrix fashion.
To drive the above-mentioned plasma addressing display device, localized discharge regions are produced by generating plasma discharge for the respective plasma chambers 106 in the order of lines. The discharge regions become the scanning unit. The respective pixels are driven by applying analog driving voltages to the signal electrodes 112 on the liquid crystal side 101 in synchronism with the line order scanning. In other words, the respective discharge regions function as sampling switches, and the analog driving voltages are sample-held at the sampled pixels.
In the above-mentioned display device, the addressing for the liquid crystal cell can be made by repeating the selective plasma discharge. The plasma discharge can be generated when a high tension voltage is applied between a pair of the discharge electrodes, that is, the anode and cathode electrodes. The discharge electrodes are formed by a metallic thin film ordinarily. Therefore, the electrodes are subjected to abrasion and corrosion due to repeated discharge. In particular, electrodes corrosion is accelerated markedly by impurities such as moisture or oxygen, and therefore the electrode quality is deteriorated, resulting in a short life time of the plasma cell. In general, the above-mentioned impurities enter the cell from the outside or are produced inside the cell.
To overcome the above-mentioned problem, conventionally, the plasma cell is sealed by frit so as to obtain air-tight plasma chambers formed of inorganic material, and additionally baked for outgassing or gas evolution. Therefore, in the prior art display device, since the plasma cells must be treated at high temperature during the manufacturing process, there exist problems in that the production cost is relatively high and further it is difficult to increase the size of the plasma cells.
On the other hand, the liquid crystal cells are usually treated at low temperature during the manufacturing process thereof. Therefore, there arise various problems in that the manufacturing process of the display devices is markedly restricted, because the liquid crystal cells are treated after the plasma cells have been assembled, as an inevitable consequence.
These problems will be described in more detail with reference to FIG. 11 which shows the prior art manufacturing process of the display device. First, the channels 105 are formed on the surface of the substrate 104 by etching technique. Secondly, the plasma electrodes such as the anode and cathode electrodes 108 and 109 are formed on the bottoms of the channels 105, respectively. Then, the dielectric partition 103 is bonded onto the substrate 104 by use of a frit sealing material 113. The partition 103 is formed of an inorganic material such as glass. The frit sealing material 113 serves to increase the air tightness performance of the plasma chambers and requires a high temperature treatment of about 400.degree. C. Thereafter, the plasma cell is baked for outgassing to remove gas from the plasma chambers also at a high temperature. Lastly, the liquid crystal cell substrate 110 is bonded to the substrate 104 by use of a second organic sealing material 114 such as a high molecular bonding agent. The hardening temperature of the bonding agent is as low temperature as about 150.degree. C., for instance, which is a low treatment temperature. Further, the liquid crystal layer 111 is filled in a gap formed between the partition 103 and the substrate 110. During this process, it is necessary to control the thickness of the liquid crystal layer 111 precisely on the order of several .mu.m to regulate the above-mentioned gap dimension at a constant value. In this case, however, it is extremely difficult to assemble the liquid crystal cell with the plasma cell which has been treated at high temperature, so that the thickness of the liquid crystal layer has a constant gap distance, with the result that the production yield is low. In this case, it may be possible to assemble the liquid crystal cell before the plasma cell is assembled. However, since the treatment or process temperature of the plasma cell is higher than that of the liquid crystal cell, the above-mentioned method is impossible in practice. All the above-mentioned various problems result from the fact that the plasma cells must be hermetically sealed.